The present invention relates to an image forming method applied to an image forming apparatus, such as a digital copier, having not only a copying function but also a facsimile function or printer function.
Conventionally, the digital copier has a scanner, printer, memory, image processing section, etc. The scanner optically reads out a document and converts its image to an electric signal. The image processing section applies a variety of image processing to an image data read out by the scanner. The memory accumulates the image data to which the image processing is applied by an image processing function. The printer subjects the image data which is stored in the memory to a laser electrophotographic process to form an image.
The digital copier thus configured can realize not only a copying function equal to that of an analog copier but also a facsimile transmitting function, facsimile receiving function and printer function. The facsimile transmitting function and facsimile receiving function are realized by having a facsimile communication section connected to a public network. The printer function is realized by having an image communication means relative to a host computer as an external apparatus.
Further, the digital copier has a memory for storing an image. For this reason, it can effectively offer various image edit functions as various paper handling options.
In the case of performing copy processing, a scanner operation and print operation are synchronized in the analog copier. For example, in the case of copying a four-page document, the document sheets are read out in a sequential page order on the analog copier and printing is done on both sides of the two sheets in the sequential page order.
In the digital copier, an image read out by the scanner can be stored in the memory. In the digital copier, the scanner and printer can be operated in a nonsynchronous fashion. In the case of copying a four-page document on both-sides of its sheets, for example, the digital copier reads out the four-page document and store corresponding images in the memory. In the digital copier, those images of pages 3 and 1 are printed on the "surfaces" of two sheets and accumulated in an automatic sheet surface inverting device. Then the digital copier receives the two sheets from the automatic sheet surface inverting device in a surface-inverted fashion and effects printing with the "reverse surfaces" of pages 1 and 3 sheets emerging as 2 and 4 pages, respectively.
In this way, the digital copier can rearrange the document sheets in a sequential printing order by storing the images in the memory. By doing so, the digital copier can effectively utilize the automatic sheet surface inverting device and, in the case of copying a larger amount of document in a larger quantity, exhibit an excellent productivity over the analog copier.
Some digital copier have a print data receiving section. In such a digital copier, a print function can be realized by printing the print data received from an external apparatus via the print data receiving section. Under this print function, various types of sheets can be supplied from a plurality of supplying trays provided in the digital copier. By doing so, the user can supply the sheets with the "surface" and "rear surface" sides being set or insert a special sheet between any given pages.
Further, the digital copier having the print function is often utilized among a plurality of users. In this case, the user selects, at a printing time, a sheet supplying tray (cassette) where a desired sheet is placed. The supplying tray of the digital copier can be selected by designating the sheet size only. In the case of designating the sheet size only, the digital copier searches the supplying trays and selects a designated sheet size. The function of searching/selecting the supplying trays on the basis of the given sheet size is called an "auto-cassette-switching" function.
Further, in the case where a surface or a rear surface is given as a result of printing, the user designates a supplying tray where sheets are placed with the surface or rear surface set. In the case of inserting a special sheet, such as a colored sheet, as a result of printing, the user designates a given supplying tray with a sheet placed as a special sheet.
In the digital copier, as set out above, various kinds of sheets are set in a plurality of supplying trays. At a time of printing, the user selects, from among those supplying trays, a given supplying tray with a desired kind of sheets are placed.
In the case where the sheet size only is designated by the auto-cassette-switching function, the digital copier selects a supplying tray by the designated size only. Even if, therefore, at the operation of the auto-cassette-switching function, the same sheet size is involved, a sheet is supplied with a surface or rear surface up or an unwanted kind of the sheet is supplied, thus presenting a problem. Further, if no sheet is left in the supplying tray to which a surface or rear surface of the sheet is designated or a special sheet is designated, a sheet of the same size will be selected under the operation of the auto-cassette-switching function, thus posing a problem.